Patent Literature 1: JP 2010-258742 A
Conventionally, an antenna and a wireless transmission portion are often connected using a coaxial cable. This connection is known to involve a loss when a signal is transmitted over the coaxial cable. In order to compensate such a loss, an input level is adjusted in Patent Literature 1.
In addition, considering mass production, a first wireless transmission portion needs to transmit a signal with an output power identical to that of any second wireless transmission portion different from the first wireless transmission portion. However, an inter-individual difference in a frequency characteristic etc. in the wireless transmission portions leads to an inter-individual difference in an output power as well. Then, in order to eliminate such an output power difference due to the inter-individual difference, each wireless transmission portion is subjected to adjustment while a resulting adjustment value is written in a storage portion.
When a subject wireless transmission apparatus is employed or mounted in each vehicle, an arrangement of an antenna and wireless transmission portion differs depending on each of different vehicle models of vehicles. The coaxial cable which connects the wireless transmission portion to the antenna also differs in length etc. depending on each vehicle model. This leads to a difference in signal losses produced in the coaxial cables of the different vehicle models. Further, in some cases, an antenna exhibits different gains depending on the different vehicle models. In contrast, a transmission output power of the wireless transmission apparatus is subject to a loss in a coaxial cable. Therefore, an adjustment value needs to be changed depending on each of the different vehicle models.
However, manufacturing a plurality of different kinds of wireless transmission apparatuses corresponding to a plurality of different vehicle models may disable a cost reduction in a mass production.